Energy storage devices are used throughout modern society to provide energy in a variety of applications, or to intermittently store and release energy, in order to stabilize power supplies. Inclusive of such electro-chemical devices are capacitors, fuel cells, and batteries. In particular, double-layer capacitors, also referred to as ultracapacitors and super-capacitors, are energy storage devices that are able to store more energy per unit weight and unit volume than capacitors made with traditional technology, for example, electrolytic capacitors.
Double-layer capacitors store electrical energy in a polarized electrode/electrolyte interface layer. Double-layer capacitors include two or more electrodes, which are separated from contact by a dielectric separator preventing an electronic (as opposed to an ionic) current from shorting the two electrodes. Both the electrodes and the porous separator are immersed in an electrolyte, which allows flow of the ionic current between the electrodes and through the separator. At the electrode/electrolyte interface, there is formed a first layer of solvent dipole and a second layer of charged species (hence, the name “double-layer” capacitor).
[With each type of energy storage device there are associated positive and negative characteristics, upon which decisions are made as to which device is more suitable for use in a particular application. The overall cost of an energy storage device is one characteristic that can make or break a decision as to whether a particular type of energy storage device is used.
In particular, as the demand for double-layer capacitors is increasing due to high-volume applications, among others, in the automotive, aerospace, hand-tool and power generation industries, a need has arisen to provide double-layer capacitors, and, in general, energy storage devices that combine a high operating reliability and increased power storage capabilities with reduced manufacturing costs.